If you're looking for a specific work of art, start with Artstor. If you don't find it, try the other databases below, or just do a Google search along with the word "exhibit" or "museum" or "archive" (so that you get an accurate, high quality image).
If you're looking for historic documents, images, newspapers, etc., public domain databases are a great place to start! Those are images that have been around for so long the copyright expired.
For great visual stories of historical topics, check out Google Arts & Culture!
ARTstor Digital Library provides more than one million images with tools for teaching and research. Its collections comprise contributions from outstanding museums, photographers, libraries, scholars, photo archives, and artists and artists' estates from all over the world. The collections encompass a wide variety of subject areas, including art, architecture, music, religion, anthropology, literature, world history, American Studies, Asian Studies, Classical Studies, Medieval Studies, Renaissance Studies, and more. ARTstor is a non-profit initiative, founded by The Andrew W. Mellon Foundation.
New York Public Library Digital Gallery
NYPL Digital Gallery provides free and open access to over 640,000 images digitized from the The New York Public Library's vast collections, including illuminated manuscripts, historical maps, vintage posters, rare prints, photographs and more.
Oxford Art Online
Oxford Art Online is an innovative gateway that offers users the ability to access and search the vast content of Grove Art Online and Oxford art reference in one location. With the 2008 complete redesign of Grove Art Online, as well as the addition of substantial new Oxford reference content, Oxford Art Online offers the most extensive and easily searchable online art resource available today, a virtual art reference library of unparalleled scope and depth.
American Journeys: Eyewitness Accounts
American Journeys contains more than 18,000 pages of eyewitness accounts of North American exploration, from the sagas of Vikings in Canada in AD1000 to the diaries of mountain men in the Rockies 800 years later. Read the words of explorers, Indians, missionaries, traders and settlers as they lived through the founding moments of American history.
American Memory Project
American Memory provides free and open access through the Internet to written and spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American experience. It is a digital record of American history and creativity. These materials, from the collections of the Library of Congress and other institutions, chronicle historical events, people, places, and ideas that continue to shape America.
Global Performing Arts Database
GloPAD (Global Performing Arts Database) records include authoritative, detailed, multilingual descriptions of digital images, texts, video clips, sound recordings, and complex media objects related to the performing arts around the world, plus information about related pieces, productions, performers, and creators.
Women Working, 1800 - 1930
Women Working, 1800 - 1930 focuses on women's role in the United States economy and provides access to digitized historical, manuscript, and image resources selected from Harvard University's library and museum collections. The collection features approximately 500,000 digitized pages and images.
World History in Context (Gale)
World History in Context is a comprehensive collection of award-winning reference, full-text articles from leading scholarly publications, an array of primary sources, images, maps and charts which provide expansive geographic and chronologic research materials for the study of world history. From ancient Europe to Latin America and from the Far East to the Renaissance, world history curricula is supported with over 1,800 primary sources, over 27 reference titles and more than 110 journals
Check out this crowdsourced list of public domain image banks, here are some of our favorites:
British Library Flickr
The British Library has published albums of public domain images on Flickr
New Old Stock
Vintage photos from the public archives. Free of known copyright restrictions. Recapturing history.
Smithsonian's collection of public domain images.
New York Times
A Wikimedia Commons site for public domain images from the New York Times.